1. Field of the Invention
The present invention relates to a method for the production of an expanded fiber-reinforced thermoplastic resin moldings.
2. Description of the Related Art
Heretofore, fiber-reinforced thermoplastic resin moldings that are reinforced with reinforcing fibers and have voids formed therein are well known as light-weight and high-strength moldings. As a method for producing such moldings, there are known, for example, a method in which a long-fiber-containing molten thermoplastic resin is charged in a mold by injection and the mold is opened by a predetermined stroke concurrently with or just before the completion of the charge, thereby foaming the inside of the resin by spring-back of the long fibers (JP 10-296772, A); and a method in which a molten thermoplastic resin containing fibers for reinforcement 2-100 mm in length is injected into a mold that can move forward and backward, a movable mold is retracted until the cavity becomes to have a volume of a molding with the injection of gas into the molten thermoplastic resin, whereby the molten thermoplastic resin is expanded by spring-back of the fibers for reinforcement (JP 10-305462, A).
However, there are problems with the former method that mold transferability is poor because the force that presses the molding against the molding surface of the mold during foaming of the long-fiber-containing molten thermoplastic resin by the spring-back force is weak or products with high expansion ratios are difficult to be obtained since the foaming is occurred only by the spring-back force. In the latter method, the pressure of the gas injected improves the mold transferability. However, there are problems with the method that an apparatus for injecting a high-pressure gas is required or it is difficult to reduce the weight of the moldings since a gas leak must be prevented by thickening a skin layer.
Under these circumstances, the inventors of the present invention studied a method for producing a high-strength expanded fiber-reinforced thermoplastic resin molding excellent in light-weight property and mold transferability.
That is, the present invention provides a method for producing an expanded fiber-reinforced thermoplastic resin molding, the method using a mold comprising a female and male dies and comprising processes for supplying a molten thermoplastic resin containing reinforcing fibers whose average fiber length is maintained at 1 mm or more to between the female and male dies to charge a mold cavity with the molten thermoplastic resin, opening a part or the whole of the mold in the thickness direction of a molding after the charging, and cooling a molding while maintaining a cavity clearance at the thickness of a final molding, the method wherein the mold cavity of the mold is interconnectable to the atmosphere, the method further comprising steps for (a) forming a solidified layer in the surface of the molten thermoplastic resin charged in the mold cavity, followed by (b) breaking at least a part of the solidified surface layer, introducing the air through the broken part concurrently with or after the breakage, and opening a part or the whole of the mold concurrently with the introduction of the air so that a cavity clearance defined there reaches the thickness of the final molding, thereby making the internal unsolidified molten thermoplastic resin expand.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d, and variations such as xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integer or step.